The display of graffiti on highway and traffic signs has become a common occurrence in many metropolitan areas. Such displays are undesirable as the graffiti can often obscure vital highway and traffic information which may create hazardous conditions and, more importantly, slows down the flow of commerce. The clean-up of graffiti and unwanted paint is very costly, time-consuming and labor-intensive. Conventional ways of cleaning graffiti and other unwanted paint using chemicals or sand blasting are not satisfactory. Chemicals are environmentally unfriendly, may be dangerous to the health of the user, and require storage of large amounts of often dangerous materials. Sandblasting harms the surface, requires the storage of sand, and leaves a pile of sand afterwards. With the limited budgets available to city planners, the graffiti is often left on the highway signs. The motorist is then left to decipher the information underlying the graffiti to avoid getting lost. Periodically, the replacement of the vandalized signs becomes necessary but this solution can be expensive and cost prohibitive for many cities.
One particularly important application is "high intensity" highway signs. These are the "FREEWAY ENTRANCE", "DO NOT ENTER" and "ONE-WAY" signs at every freeway entrance and exit. They contain internal retro-reflectors that make them tremendously bright when observed with automobile headlights. The signs must be placed only a few feet off the ground, so that they are easily observable by drivers. Graffiti destroys their retro-reflectivity and, because they are placed so low, they are easily vandalized. Chemical paint removers must be used when the paint is very fresh, and these removers degrade the signs sufficiently that they may be used only a few times. Sand blasting cannot be used because it destroys the retro-reflection. The only other solution used now is to remove and replace these signs: an expensive proposition.
What is needed is a system to remove graffiti that is environmentally friendly, has very few expendables, is safe for the operator, produces minimum degradation of the surfaces and leaves very little mess at the work-site. At the same time, the cleaning method must be cost-effective. This requires an apparatus with very little set-up and take-down time and minimal labor effort. Ideally, the operator would not have to touch the surfaces at all.
Such a system could be used on highway signs, concrete blocks that surround highways, brick and marble buildings and other structures, stones, trees, and unpainted wooden structures. The system could also be used to remove unwanted paint from selected portions of furniture and other antiques.
With the advent of laser technology, lasers have been used to remove paint from objects such as aircraft or boat hulls. Laser technology systems for such applications have been disclosed that use different sensory devices to detect the amount of paint removed from a given surface. These techniques, while adequate for their intended purpose, preparing the surface of an aircraft or boat for subsequent repainting, do not disclose an adequate system for the restoration of highway signs or other surfaces covered by paint in localized areas.
Hand-held solid-state lasers have been used to clean marble statues and buildings and to remove grime and coal smoke. A Q-switched YAG laser for this use is available for purchase from Hedge Clemco in England. It uses hand-held fiber delivery system, which is awkward and undesirable where the height of the highway sign may be outside the reach of the operator or where more rapid, automated removal is needed.
Pulsed lasers, including Q-switched lasers, have been used to remove portions of integrated circuits in semiconductor devices (laser resistor trimming). Here, the part is usually moved under a fixed laser beam and one or a few blasts are used to remove the unwanted material. There is generally no scanning of the laser beam and there is no interactive control between the position of the unwanted materials and the direction of the laser beam. This application has allowed scientists to understand the process of "ablation" in which the unwanted material is removed so fast that the substrate does not heat up.
Thus, the need exists for a low-cost alternative to conventional graffiti-cleaning techniques in order to restore and extend the life of the highway signs and to remove graffiti easily from bridges, walls and highway abutments. The need also exists for a laser cleaning system that minimizes the damage to the reflective properties of the signs, thereby ensuring their continued service to motorists.